In the industry where machinery is involved in the production of goods, there is always a probability that a piece of metal, such as a screw or a bolt, does break away from the machinery, and finally ends up in the processed product. Therefore metal detection systems are used at various stages of a production process to detect products that are contaminated by metal. Metal detection systems are also often used for inspecting finished product, in order to ensure consumers' safety and quality standards.
A known metal detector used for detecting metallic presence in products generally can include an electromagnetic coil arrangement on a former or also called a frame assembly having an pass-through aperture. The former assembly is surrounded by a housing or case and the remaining space between the former assembly and the case is filled with a potting medium.
A metal detection system as mentioned above includes a former assembly with an electromagnetic coil arrangement that can include three coils, one transmitter coil and two receiver coils that are aligned in parallel. Since the two receiver coils are identical and at the same distance from the transmitter coil, an identical voltage is induced in each of the receiver coils. When the receiver coils are connected in opposition, these voltages cancel out resulting in a zero output when there is no product present in the metal detector. This is a balanced coil system. During the inspection process, the product, for example transported on a conveyor belt, is passed through the coils of the balanced coil system. In the transmitter coil which can be placed between the receiver coils, flows an electrical current that generates an alternating magnetic field that induces an electrical signal in the two receiver coils. In most metal detectors like mentioned above, the receiver coils are positioned symmetrically to the transmitter coil, so that identical signals are induced in both receiver coils when no product is present in the balanced coil system. In addition, the receiver coils are coupled together in such a manner that the signals induced therein are subtracted from each other. In that way, when no product is present in the balanced coil system, there is a zero signal at the output of the receiver coils. However, a piece of magnetically and/or electrically conductive material that passes through the balanced coil system will disturb the magnetic field and will cause modifications of the electrical signal that is induced in the receiver coils. These perturbations occur first in the first receiver coil and then in the second receiver coil, when the product approaches it.
Metal detector assemblies are known which include a coil-carrying former or a plurality of laminated frames that form the former between which the transmitting and receiving coils are located surrounded by a case or housing. Such assemblies can be complicated and costly to manufacture; additionally and/or alternatively they can be subject to tension due to the instability between the case, former and the coils. Such instability in the assembly can lead to issues such as electronic drift and reduction in repeatability which in turn affects the yield of the detector.
To ensure a stable design and construction of a metal detector, it is of high importance that the physical connections made between the various components of a metal detector have to be stable avoiding vibrations and cause minimum if not eliminate the tension between them.
The document WO 95/11462 A1 discloses a method of joining the case and the former of a metal detector. The method includes using mechanical fixings like aluminum dowels. These aluminum dowels, however, themselves not being a part of the case or former (it is an external entity) introduce a mechanical tension between the case and the former in case of vibrations or shocks. In an another embodiment, the document discloses a tongue-groove joint to join the parts of the case to each other and also the case to the former in certain adjoining parts. As mentioned in the document, the tongue-groove joint is intended to provide a minimum magnetic leakage from the joint faces of the case. The document however, does not disclose measures taken to ensure the tension free physical connection or physical joint between the former and the case which in turn affects the stability of the metal detector.
The document GB 2267351 A discloses a method for assemblying a metal detector. In the document, the former is constructed from three frames and held together with the use of a glue or an interlock, after which the coils are arranged on the former. A layer of plastic coating which acts like a potting medium is formed on the former followed by an application of a metal layer coating using methods like metallic painting, arcspraying, plating, or vacuum depositing. This metal layer coating upon hardening forms the case or the housing of the metal detector. The disclosed method in terms of case construction uses methods like metallic painting, arcspraying, that can be expensive in terms of resources and labour.
It is known in the art that the stability between a former and a case can be further ensured by introducing a potting or a plastic medium in the empty space between the two. The potting medium used can be resin or foam or in some cases concrete, which holds the assembly together. Examples of using a potting medium for the above purpose are discussed in documents GB 2267351 A, GB 2232254 A and WO 95/11462 A1. However, the contact surface between the former and the potting medium is limited to a flat horizontal plane which leads to a weak connection between the two. In presence of mechanical vibrations or shocks, the potting medium may move with respect to the former due to the weak contact surface area between them.
In addition, known formers can have smaller widths than that of the case. In case of such formers with a small width, they can be covered with a potting medium and then encompassed by a case, increasing the chances of instability.
Metal detectors can have pass-through apertures lined with a conductive material like an electrostatic screen to protect the coils from a phenomenon called bulk effect, the more scientific term of which is capacitive effect. It is known that at low operating frequencies, the electrostatic screen is applied across each side of the pass-through aperture leaving a small gap to the edge of the case to ensure no contact with the case in order to avoid intermittent eddy currents to flow through the screen picked up by the metal detector receiver coils creating unwanted nuisance detection signals. Further, at higher operating frequencies, it becomes necessary to remove the gaps and hence the electrostatic screen is fitted directly across onto the case. However, although this is effective as a screen, it presents a risk to the stability of the metal detector. Any cracks appearing at the screen transition onto the case has the potential to cause the abovementioned issues.